Nickel-based electrodes are commonly used in rechargeable electrochemical cells. For example, nickel hydroxide particles, Ni(OH).sub.2, usually constitute the positive electrode in both nickel-cadmium ("nicad") and nickel-metal hydride cells. Ni(OH).sub.2 is the material of choice for positive electrodes in both types of cells because it can offer high energy density as well as good rate capability, desirable properties in today's battery market. High energy density is obtained through use of pasted electrodes in which a paste comprising high density spherical Ni(OH).sub.2 particles is applied to a foam substrate; high rate performance is typically obtained by using sintered Ni(OH).sub.2 electrodes.
However, in spite of these advantages of Ni(OH).sub.2, the changing structure of this material, in response to electrical charging and discharging, presents problems for designing a rechargeable cell having a commercially desirable life. The crystal structure of Ni(OH).sub.2 is characterized by a hexagonal unit cell with a layered structure comprising one nickel, two oxygen, and two hydrogen atoms per cell, as illustrated in FIG. 1. This ".beta.-Ni(OH).sub.2 " layered structure can also be described as a system of lamellar plates comprising an arrangement of nickel and oxygen atoms. When the typical .beta.-Ni(OH).sub.2 electrode is charged, the positive electrode is oxidized and the Ni(II) of .beta.-Ni(OH).sub.2 releases one electron to become Ni(III) and form beta nickel oxyhydroxide, .beta.-NiOOH. In .beta.-NiOOH, the lamellar plates of the crystal become slightly displaced away from each other, changing the volume of the unit cell. Upon discharge, the positive electrode is reduced, the Ni(III) of .beta.-NiOOH accepting one electron to convert back to Ni(II) and form .beta.-Ni(OH).sub.2, whereby the plates return to their initial positions.
At relatively higher discharge rates (e.g., 5-10 C), this reduction of Ni(III) to Ni(II) occurs more readily at the surfaces of the Ni(OH).sub.2 particles than within the bulk of these particles. This difference has a marked effect especially in high drain rate applications, such as power tools, where the net time allowed for discharge of the electrode is short. There, at least some of the Ni(III) situated within the bulk of the particles does not have sufficient time to become reduced back to Ni(II). As a result, these areas may begin the next charging cycle in the Ni(III) state, and so are subject to an even greater degree of oxidation. Thus, where the cycle of recharging and quick, high-rate discharging is repeated, these areas of Ni(III), i.e. .beta.-NiOOH, typically begin converting to .gamma.-NiOOH, a material comprising both Ni(III) and Ni(IV), e.g., in the form of species including nickelate, (NiO.sub.2).sub.3.sup.-, in which the nickel atoms have fractional formal valences such as 3 2/3.
In forming .gamma.-NiOOH, the lamellar plates of the crystal become significantly displaced away from each other, greatly expanding the crystal volume. FIG. 2 illustrates the differences in crystal structure among .beta.-Ni(OH).sub.2, .beta.-NiOOH, and .gamma.-NiOOH. Upon discharge, .gamma.-NiOOH converts back to .beta.-Ni(OH).sub.2. This charge-discharge series thus creates an extreme expansion-contraction cycle which cracks the crystalline structure of the electrode particles so as to create many different particles thereby increasing the porosity of the electrode. Also, as this cracking process continues, many smaller particles are formed, causing the total particle surface area of the electrode to greatly increase. The greater surface area and increased porosity so produced result in migration of the electrolyte into the electrode and away from the cathode-anode separator so as to foster the formation of "dry" areas therein. These "dry" areas within the separator increase the internal resistance of the cell, thereby leading to generation of heat during charging and oxidation of the separator. Over the course of repeated charge-discharge cycles, these cracking, drying, and pressurizing processes degrade the positive electrode, causing cells to prematurely fail.
As a result, these processes are responsible for the "rainbow" shape of the typical graph of cell capacity versus charge-discharge cycle number in nickel electrode cells as illustrated in FIG. 3. Thus, a cell rated as having an initial capacity of, e.g., 1700mAH ("milliamp-hours")--based on a single-electron transfer--will, as .gamma.-NiOOH accumulates, develop an increased capacity which may reach a maximum of, e.g., 2000 mAH or more. Then, once the amount of .gamma.-NiOOH reaches a threshold level, the 2000 mAH capacity will decrease as premature degradation occurs, leading to cell failure. Such premature cell failure and variation in capacity are undesirable in the battery market.
In response to these problems, rechargeable battery makers regularly use Ni(OH).sub.2 which has been produced by co-precipitation with anti-.gamma. additives, such as cadmium compounds. The additives interfere with .gamma.-NiOOH formation, apparently by occupying the spaces between lamellar plates of the .beta. crystal structures and interacting with the plates to largely inhibit their extensive displacement to the .gamma. structure.
However, this approach increases the materials input and cost of the electrode and fails to address the core problem of the difference in discharge rates of surface and subsurface nickel, thus limiting the degree of improvement in electrode charge-discharge efficiency which may be obtained thereby. Moreover, even with these additives, given sufficient overcharging, .gamma.-NiOOH may to some degree still form within the electrode. Therefore, there is a need for an electrode which is more resistant to formation of and degradation by .gamma.-NiOOH.